Prepare v2.1.0 open-source release

[weiwill88]

Summary

• add an MIT license, bilingual project-first documentation, and a current UI screenshot
• add contribution, security, conduct, issue, and pull request guidance
• add credential-free tests and GitHub Actions CI for configuration, API status, document loading, BM25, hybrid retrieval, and missing-key behavior
• remove commercial book, course, community, and store promotion
• fix Gradio 6 theme initialization discovered during browser verification

Validation

• Python source compilation passed
• 8 tests passed locally on Python 3.12 with 34% baseline coverage
• Gradio 6.23.1 UI returned HTTP 200 and rendered successfully in Playwright
• no external model keys were used

Release

This PR prepares the repository for the v2.1.0 release described in CHANGELOG.md.